This invention relates to apparatuses and methods for conducting gas chromatography separation procedures, and particularly to mechanical inlet systems for such apparatuses, and approaches toward multiplexing multiple columns.
Gas chromatography is a widely employed technique for the separation and analysis of complex mixtures of volatile organic and inorganic compounds. The analyte mixture is separated into its components by eluding them from a column having a sorbent by means of moving gas.
Gas chromatography procedures can be classified into two major divisions: gas-liquid chromatography, and gas-solid chromatography. Gas-liquid chromatography is presently the most widely employed type and incorporates a nonvolatile liquid sorbent coated as a thin layer on an inner support structure, generally the inside surface of a capillary tube. The moving gas phase, called the carrier gas, flows through the chromatography column. The analyte partitions itself between the moving gas phase and the sorbent, and moves through the column at a rate dependent upon the partition coefficient or solubility of the analyte components. The analyte is introduced at the entrance end of the column within the moving carrier gas stream. The components making up the analyte become separated along the column and escape from the exit end of the column at intervals and in concentrations characteristic of the properties of the analyte components.
A detector, for example, a mass spectrometer, thermal conductivity detector or flame ionization detector (FID) at the exit end of the analytical column responds to the presence of analyte components. Upon combustion of the eluded material at an FID, charged species are formed in the flame. The flame behavior is monitored through a biased ion detector which, along with associated electronics, produces a chromatogram which is a time versus magnitude trace of the detector output. The trace for a complex analyte mixture includes numerous peaks of varying intensity. Since individual constituents of the analyte produce peaks at characteristic times and whose magnitude is a function of their concentration, much information is gained through an evaluation of the chromatogram.
Today there is an increased emphasis toward so-called "fast gas chromatography" or "fast GC". Applications include process stream monitoring, environmental monitoring, and IC engine exhaust gas analysis. Ideally, such a system would be capable of performing an analysis within several seconds, which using conventional approaches, would take several minutes or more. Increasing the speed of analysis can be achieved by providing a relatively short separation column, or by using other techniques for causing components of interest to traverse the column more quickly. In order to provide useful information, the individual analyte components must elute separately at the detector, thus producing distinct peaks. As the length of time over which the sample is injected at the inlet end of the separation column (injection time) increases, the peaks produced by elution of the components tend to broaden, smear, and overlap. It is, therefore, essential that a short duration sample "plug" be presented at the column inlet during injection in order to provide gas chromatography evaluation in a short period of time while providing acceptable resolution.
Various types of injection systems are presently known for placing a sample at the inlet end of a separation column. One approach uses a mechanical valve, which is controlled to intermittently communicate a sample stream with the analytical column as a sample plug. Previously known injection valves, however, have significant limitations in terms of their minimum injection time. Known mechanical valves also have shortcomings in terms of mechanical wear and contamination of the sample stream caused by the presence of lubricants and other impurities within the valve. In addition, valve elements can become coated with a sample which is retained and mixed with subsequent samples, and thus the system has an undesirable artifact termed a "memory" effect.
One facet of the present invention is to provide improved mechanical injection valves. The injection valves of this invention include a shuttle element which, in two described embodiments are moveable linearly, and in another embodiment moves rotationally. The shuttle has a small exit port through which a sample stream is discharged. The valves of this invention cause the shuttle exit port to be swept rapidly across an inlet of an analytical column as a means of creating a narrow sample plug. Carrier gases are routed through the device in a way that the sample stream is continually vented and the area of the column inlet is flushed with carrier gas between injections. It is believed that the injection valves in accordance with the present invention are capable of generating sample plugs as narrow as several milliseconds in duration. The injection valves of this invention may also be used for injecting a sample onto multiple columns in a programmed manner. Moreover, the valves may be used in the reverse sense for coupling multiple sources to a single outlet or column.
Another consideration which complicates efforts toward reducing analysis time is the result of the fundamental conflict between resolution of separated components and separation time. In general, as separation time increases, better separation of the individual constituents of an analyte mixture occurs, producing well defined distinct chromatograph peaks which do not overlap those of other components. In some instances, a mixture may contain components of interest having a relatively high boiling point which tend to elute well after initial peaks are produced. For such an analyte, if separation time is shortened and it is desired to evaluate the high boiling point components, the low boiling point components which elute faster tend to produce smeared and overlapped peaks with poor definition. In order to increase resolution of the low boiling point components, overall analysis time needs to be increased.
In accordance with the present invention, a high degree of resolution of output for specific mixture constituents is provided by using a multiple column system in which samples are injected into columns having differing separation characteristics. Such differing characteristics can be achieved through the use of columns having varying lengths, diameters, liquid or solid phase materials, carrier gas velocities, etc. Moreover, the speed of separation can be controlled by the temperature of the separation columns. Analyte eluted from the multiple columns is preferably evaluated at a single detector which produces a chromatogram which is a composite of outputs from the multiple columns. The characteristics of the columns and the injection sequence are preferably chosen so that significant peaks can be superimposed relative to the output of other columns in a single chromatogram which provides good resolution of all components of interest. The portions of a single column chromatogram which are ordinarily free of peaks can be used as an interval in which to insert peaks from another column related to specific compounds of interest. In this way a complex chromatogram including various constituents of an analyte can be created through an overlapping of the outputs of multiple columns. The system would, therefore, provide high resolution of relatively high boiling point components while providing fast overall analysis time and preserving resolution of relatively low boiling point components of a mixture. These advantages can be achieved through the use of a single detector which, in modern gas chromatography systems, may be a very costly component of the overall system.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.